Multi-parameter eye axis measurement system based on self-mixing vernier effect

Abstract

The invention discloses a multi-parameter eye axis measurement system based on a self-mixing vernier effect, and relates to the field of optical fibers. A light beam emitted by the low-coherence light source is divided into optical fiber reference light and optical fiber measuring light through the coupler; the optical fiber measuring light outputs space measuring light through a first circulator, and the space measuring light scans eyeballs through a zoom lens; the optical fiber measurement light enters the optical fiber delay line through the second circulator and the wavelength division multiplexer, and is output from the reflection end of the second circulator after being reflected; the space measurement light is reflected by the eyeball and then first low-coherence light is output from the reflection end of the first circulator, and the sample measurement light passes through the reflection end of the second circulator and then second low-coherence light is output; the two beams of low-coherence light generate interference through a second coupler; light beams emitted by the high-coherence light source are subjected to self-mixing interference in the phase-shifting grating and serve as a scale for low-coherence measurement, and the eyeball diopter is calculated by collecting a Hartmann array diagram. According to the scheme, the signal-to-noise ratio of data calibrated and measured by using the optical fiber and the self-mixing interference method is higher, and the data is more accurate.

Description

Technical Field

[0001] The embodiments of the present application relate to the technical field of optical fibers, and in particular to a multi-parameter eye axis measurement system based on the self-mixing cursor effect. Background Art

[0002] At present, human health problems have gradually become the focus of the whole society, and ophthalmic problems are particularly emphasized. Most ophthalmic problems can be reflected in the axial parameters and refractive power of the eyeball. The monitoring of eye axis parameters and refractive power can help ophthalmologists timely understand the changes in the eyeball structure and guide the treatment and management of related diseases. The eye axis parameters mainly include eye axis length, lens thickness, corneal thickness, anterior chamber depth, etc. The low-coherence light interference technology combines the confocal microscopy technology and the optical coherence tomography (OCT) technology with the advantages of white light coherence imaging, and can non-invasively achieve micron-level resolution measurement in biological tissues, effectively preventing the occurrence of secondary infections and other advantages. It has gradually become the mainstream technical solution for non-contact and high-precision eye axis parameter measurement.

[0003] However, in the related technologies, most of the eye axis biometric instruments adopt a spatial optical path structure. The detection of the distance between the measured low-coherence peaks requires calibrating the position of the scanning delay line with a clock system, which limits the utilization rate of the scanning beam and the integration degree of the system, and affects the accurate measurement of the signal-to-noise ratio and sensitivity. Finally, the measured eyeball parameters cannot meet the high-precision requirements. Summary of the Invention

[0004] The embodiments of the present application provide a multi-parameter eye axis measurement system based on the self-mixing cursor effect to solve the problem of inaccurate measurement of traditional measurement systems. The system includes: A low-coherence light source, the beam emitted by the low-coherence light source is divided into a fiber reference light and a fiber measurement light through a first coupler; the fiber measurement light outputs a spatial measurement light through a first circulator and a collimator, and the spatial measurement light focuses the scanning beam on different parts of the eyeball through a zoom lens; the fiber measurement light enters a fiber delay line through a second circulator and a wavelength division multiplexer, and is reflected from the fiber delay line and output from the reflection end of the second circulator; A high-coherence light source, the beam emitted by the high-coherence light source passes through an erbium-doped fiber connected with a phase-shifting grating, is reflected by the wavelength division multiplexer and the fiber delay line, and self-mixing interference occurs at the phase-shifting grating to calibrate the low-coherence peak spacing; the spatial measurement light is reflected by the eyeball and then passes through the collimator and is output as a first low-coherence light from the reflection end of the first circulator, and the sample measurement light is output as a second low-coherence light from the reflection end of the second circulator; the two low-coherence lights generate interference through a second coupler, and the interference data is received by a balanced photodetector, and the eyeball data is obtained by combining the measurement of the low-coherence peak spacing; The Hartmann microlens array is such that the spatial light reflected by the eyeball is incident on the Hartmann microlens array and then focused on the photosensitive surface of the CCD. The refractive power of the eyeball is calculated through the collected Hartmann array diagram.

[0005] Specifically, a two-dimensional galvanometer is provided in the spatial optical path between the zoom lens and the eyeball, and the spatial light is focused on different regions of the eyeball through the two-dimensional galvanometer; The two-dimensional galvanometer includes an X-galvanometer and a Y-galvanometer that are perpendicular to each other. The light beam completes one cycle of X-Scan scanning after passing through the X-galvanometer, is reflected to the Y-galvanometer, and then undergoes one cycle of deflection to achieve multiple X-Scan scans, focusing the light beam on the lens, vitreous membrane, fundus, and anterior segment interface of the eye tissue to be measured for two-dimensional scanning.

[0006] Specifically, a beam splitter prism and a reflector are provided in the spatial optical path between the zoom lens and the two-dimensional galvanometer to introduce a part of the light beam reflected by the eyeball into the Hartmann microlens optical path, and the refractive power of the eyeball is calculated through the Hartmann array diagram collected by the Hartmann optical path.

[0007] Specifically, the signal intensity received by the balanced photodetector I is expressed as follows:

[0008]

[0009]

[0010] wherein and respectively represent the central wavelength and the full width at half maximum of the spectral width of the low-coherence light source, represents the energy of the received interference signal I ; represents the sum of the DC components of two low-coherence electrical signals, represents the change amount after interference; represents the change amount of the reference light phase; During the process of measuring the signal intensity I the internal attitude of the fiber optic delay line is changed to change the reference light phase , and the eye axis parameters are inversely deduced based on the magnitude of the adjusted attitude.

[0011] Specifically, based on the fact that the interference signal generated by the high-coherence light source shows periodic changes, the D value of the low-coherence peak spacing is determined by comparing with the interference peak of the interference signal generated by the low-coherence light source, and is expressed as follows:

[0012] wherein is the wavelength of the high-coherence laser, is the air at the wavelength Refractive index below is the number of complete interference fringes, is the phase difference of the incomplete interference fringes, is the overall phase difference.

[0013] Specifically, the high-coherence light source, phase-shifting grating, erbium-doped fiber, wavelength division multiplexer, and fiber optic delay line form a three-mirror cavity model. The phase-shifting grating is equivalent to the first and second reflectors of the three-mirror cavity model, and the fiber optic delay line is equivalent to the third reflector. The adjustment attitude of the fiber optic delay line is determined according to the self-mixing interference formula , as the signal scale for low-coherence interference, is expressed as follows:

[0014] represents the group refractive index, represents the linewidth broadening factor, represents the cavity length of the equivalent resonant cavity of the phase-shifting grating, coupling efficiency, and respectively represent the optical frequency and the critical frequency, represents the speed of light, represents the equivalent external cavity round-trip delay between the phase-shifting grating and the fiber optic delay line.

[0015] Specifically, the high-coherence light source is pumped at 980 nm. A wavelength division multiplexer is provided on the fiber between the pump and the phase-shifting grating and is connected to a photodetector. The signal scale is determined by detecting the self-mixing interference signal through the photodetector.

[0016] Specifically, the refractive power of the eyeball is calculated by collecting the Hartmann array diagram, including: When the wavefront to be measured is received by the microlens array, it is divided into countless sub-wavefronts by the microlens array and converges into a spot array diagram containing the information of the wavefront to be measured on the CCD; Based on the spot array diagram, the offset of the spot position is measured; Based on the offset of the spot position, the refractive power and divergence data are calculated.

[0017] Specifically, in the process of measuring the offset of the spot position, five groups of two-dimensional surfaces are constructed based on the Zernike polynomial, and the partial derivatives of the surfaces are determined as the offsets of the positions of each sampling light point. The surface polynomial and the partial derivatives are expressed as follows:

[0018] Obtain at least one set of light points offset of , according to the reference point coordinates invert the coefficient values of the five polynomials; the offset is expressed as follows:

[0019] The partial derivative of the sampling point is expressed as follows:

[0020] Among them, represents the wavefront function, and represent the offset values, and are the corresponding derivatives, is the focal length of the lens, and n takes the value of 5.

[0021] Specifically, according to the correspondence between the partial derivative of the sampling point and the matrix, list the polynomial coefficient identity, which is expressed as follows:

[0022] Use the inverse matrix to obtain the Zernike coefficients corresponding to the wavefront aberration, which is expressed as follows:

[0023] Through the five coefficients Calculate the diopter and divergence, which is expressed as follows

[0024]

[0025] represents the diopter value, represents the divergence value.

[0026] The beneficial effects brought by the technical solution provided by the embodiments of the present application at least include: In this solution, the measurement of various parameters of the eye axis is realized through time-domain low-coherence interference. The time-domain low-coherence interference measurement system is composed of a laser, a reference arm, a measurement arm, a delay device and a detector. The measurement light carries the information to be measured and interferes with the reference light to generate interference peaks on the surfaces of various eye tissues. Finally, combined with the high-coherence interference technology, using the high-coherence interference signal as a scale, the accurate calibration of the position information of the eye axis tissue can be realized. The interference measurement of this solution reduces the complexity of the external optical path and has strong tolerance to environmental interference, which is suitable for high-precision and miniaturized sensor applications. The introduced diopter measurement system part composed of the Hartmann lens group can, while measuring the eye parameters, additionally split the light to measure and obtain the diopter and divergence data of the eye, improving the system function. Brief Description of the Drawings

[0027] Figure 1 It is a schematic structural diagram of a multi-parameter eye axis measurement system based on the self-mixing cursor effect provided by an embodiment of the application; Figure 2 It is a theoretical schematic diagram of a three-mirror cavity model constituted by using laser feedback interference technology; Figure 3 It is a schematic structural diagram of an axial length high-resolution measurement system that combines high and low coherence measurement and filtering design provided by another application embodiment; Figure 4 It exemplarily shows a schematic diagram of a Hartmann array diagram; Figure 5 It lists schematic diagrams of the time-domain diagram of the original data and the frequency-domain diagram of the original data; Figure 6 It shows a possible FIR band-pass filter diagram; Figure 7 It is a possible Hilbert envelope diagram listed; Figure 8 It is the frequency-domain diagram of the envelope after data processing; Figure 9 It shows a possible filter effect diagram. Detailed implementation manners

[0028] To make the objectives, technical solutions, and advantages of the present application clearer, the following will further describe the embodiments of the present application in detail with reference to the accompanying drawings.

[0029] "Multiple" mentioned herein refers to two or more. "And / or" describes the association relationship of associated objects and indicates that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone. The character " / " generally represents an "or" relationship between the front and rear associated objects.

[0030] Figure 1 It is a schematic structural diagram of a multi-parameter axial length measurement system based on the self-mixing cursor effect provided by an embodiment of the present application. As a whole, it includes a low-coherence interference measurement system part constituted by a low-coherence light source, a circulator, and a coupler, a high-coherence auxiliary measurement system part constituted by a high-coherence light source and an optical fiber delay line, etc., and a diopter measurement system part based on a Hartmann lens group. In addition, it also includes a balanced photodetector for measuring interference data, a filtering processing algorithm after photoelectric conversion, etc. After measurement and filtering processing by the balanced photodetector, relevant eye data can be obtained.

[0031] The low-coherence light source is responsible for emitting light beams of corresponding wavelengths. Its output is connected to the first coupler through an optical fiber. The output of the first coupler is divided into two paths, which are respectively connected to the first and second circulators through optical fibers. The direct output end of the first circulator is connected to a collimator through an optical fiber. After passing through the collimator, it is converted into spatial light for transmission. The direct output end of the second circulator is connected to a wavelength division multiplexer (i.e., wavelength division multiplexer 2) through an optical fiber, and the output end of the wavelength division multiplexer is connected to an optical fiber delay line. The spatial measurement light is reflected by the eyeball and then passes through the collimator and the first low-coherence light is output from the reflection end of the first circulator. The sample measurement light is output as the second low-coherence light from the reflection end of the second circulator. The two low-coherence lights generate interference through the second coupler, and after interference, it is detected by a balanced photodetector.

[0032] The description of its optical path is as follows: The light beam emitted by the low-coherence light source is divided into an optical fiber reference light and an optical fiber measurement light through the first coupler. The optical fiber measurement light passes through the first circulator and the collimator to output spatial measurement light, and the spatial measurement light focuses the scanning beam on different parts of the eyeball through a zoom lens. The optical fiber measurement light enters the optical fiber delay line after passing through the second circulator and the wavelength division multiplexer, and is output from the reflection end of the second circulator after being reflected by the optical fiber delay line.

[0033] In some embodiments, light with a wavelength of 850 nm emitted by the low-coherence light source passes through a 95:5 coupler 1 and is divided into two beams of light that can interfere with each other. The polarization directions of these two beams of light are the same and the frequencies are the same. Subsequently, these two beams of light are reflected by different reflecting surfaces respectively and return to coupler 3 to generate interference. Within the interference distance, when the phase difference between these two light waves is 0 or an integer multiple, the light intensity of the superimposed wave is the maximum. By changing the position of the reference arm to adjust the phase of the reference light, so that it sequentially generates the maximum interference light intensity with the light reflected by different eye tissues in the measurement arm. In this way, the eye parameters such as the axial length of the eye, the corneal thickness, and the anterior segment of the eye can be deduced by the moving distance of the reference arm. Here, the reference arm and the measurement arm respectively refer to the output path parts of the two circulators. In Figure 1 it, the reference arm is the optical fiber delay line after the output of circulator 2; the measurement arm is the spatial optical path device components after the collimating lens and the eyeball part.

[0034] Considering the measurement accuracy, the system uses a low-coherence light source, and the light source has a certain bandwidth. The signal received by the detector is the interference superposition of all wavelengths within the bandwidth. Therefore, the signal intensity I received by the balanced photodetector is the superposition of the interference intensities of each wavelength, which can be expressed as:

[0035]

[0036]

[0037] Among them, and respectively represent the central wavelength and the full width at half maximum of the spectrum of the low coherence light source, represents receiving the interference signal I energy; represents the summation of the DC components of two low coherence electrical signals, represents the change amount after interference; represents the change amount of the reference light phase.

[0038] Because the eye parameters are deduced by the moving distance of the reference arm, that is, the internal attitude of the fiber optic delay line is changed during the process of measuring the signal intensity I The purpose is to change the reference light phase , and finally various eye parameters are deduced based on the adjusted attitude data and the measured relevant parameter values.

[0039] In order to meet the requirements of low coherence measurement accuracy and scanning speed, we use a fiber optic delay line to achieve high-precision and high-frequency scanning of the reference arm. In addition, in order to obtain an accurate phase change amount, this system introduces a high coherence interference technology to assist the measurement. This application selects a fiber laser as the high coherence light source. An erbium-doped fiber connected with a phase shift grating is connected to the high coherence light source, and is sent into the wavelength division multiplexer 2 through the erbium-doped fiber and then input into the fiber optic delay line. That is, the fiber optic delay line is the common part of the high coherence and low coherence optical paths. After the reflection of the fiber optic delay line, it will interfere with the input light beam, and the phase shift grating is located at the interference position. The interference generated at the phase shift grating can calibrate the low coherence peak spacing based on the Vernier effect.

[0040] Specifically, when the reference arm is scanned, the low coherence interference optical path can detect the interference peaks of different optical surfaces, and the high coherence signal intensity will show periodic changes. By comparing these two interference signals, the phase difference between the low coherence peak signals can be obtained. By comparing with the interference peaks of the interference signals generated by the low coherence light source, the adjacent low coherence peak spacing D value can be determined, which is expressed as follows:

[0041] Among them, is the wavelength of the high coherence laser, is the refractive index of air at the wavelength , is the number of complete interference fringes, is the phase difference of the incomplete interference fringes, is the overall phase difference.

[0042] Considering that high coherence technology is used in some experiments, and its interference vernier effect and precision are limited. Therefore, in order to further improve the integration and sensitivity and reduce the complexity of the optical path in this application, the self-mixing effect is utilized to generate interference. That is, a three-mirror cavity model is composed of a coherent light source, a phase-shifting grating, an erbium-doped fiber, a wavelength division multiplexer, and an optical fiber delay line. The phase-shifting grating is equivalent to the first and second reflectors of the three-mirror cavity model, and the optical fiber delay line is equivalent to the third reflector. The adjustment attitude of the optical fiber delay line is determined according to the formula for generating self-mixing interference , as the signal scale of low-coherence interference

[0043] Figure 2 is a theoretical schematic diagram of a three-mirror cavity model formed by using the laser feedback interference technology. M1, M2, and M3 represent the three reflectors respectively. After the laser Laser is emitted, the beam is emitted through M2, reflected by Target at M3, and then returns between M1 and M2, where self-mixing interference is generated with the incident beam represents the effective refractive index of the inner cavity of the phase-shifting grating represents the effective refractive index of the outer cavity

[0044] Considering the output mirror of the laser, that is, the intensity reflectivity of the second reflector M2 is , where R2 represents the reflectivity of this reflector. The influence of the third reflector M3 on the second reflector lies in changing the reflectivity and phase of the second reflector. The reflectivity after being affected by the third mirror of the outer cavity on the second mirror can be expressed as:

[0045] Here represents the coupling efficiency represents the optical frequency and represents the round-trip delay of the outer cavity. At the same time, the stability condition for forming the laser resonator still needs to be satisfied, that is is a positive real number. Since the laser resonator still needs to be in a relatively stable state, then the modulus of the imaginary part of is a small quantity. When the above relatively stable conditions are satisfied, there is , and it is generally defined that .

[0046] Among them, when , can be obtained . Since the laser still maintains stable output, the phase change for one round trip inside the laser is 2π. Therefore, it can be obtained that:

[0047] Among them is the eigen wavenumber of the laser resonator is the cavity length of the laser resonator, is the effective refractive index of the inner cavity of the laser, is an integer. Since the effective refractive index of the inner cavity is , so we get:

[0048] Consider the influence of the change of the optical field on the group refractive index. The group refractive index is , and it is replaced by the linewidth broadening factor to obtain the adjusted attitude (displacement information) as the signal scale of low-coherence interference, which is expressed as follows:

[0049] represents the group refractive index, represents the linewidth broadening factor, represents the cavity length of the equivalent resonator of the phase-shift grating, coupling efficiency, and respectively represent the optical frequency and the critical frequency, represents the speed of light, represents the equivalent round-trip delay of the external cavity between the phase-shift grating and the fiber optic delay line

[0050] In some embodiments, the eye parameters that can be measured by the sample arm of the spatial light part are determined according to the actual situation. Since the distance from the cornea to the retina exceeds 20 mm, the galvanometer scan can only scan the images near the sample imaging point. If the detection light cannot be well focused on the tissue structure level in the eye to be measured, it will reduce the intensity of the light signal reflected by the sample arm, and further reduce the intensity of the obtained coherent light signal, thus affecting the detection signal-to-noise ratio. Therefore, all imaging data cannot be obtained through a single scan. For example Figure 1 in the system in order to achieve high-contrast and high-resolution fundus and anterior segment images, the present application additionally sets a two-dimensional galvanometer in the spatial optical path between the zoom lens and the eye. The spatial light is focused on different regions of the eye through the two-dimensional galvanometer and reflects the eye data of different regions. The two-dimensional galvanometer includes an X-galvanometer and a Y-galvanometer perpendicular to each other. The light beam completes a cycle of X-Scan scanning through the X-galvanometer, is reflected to the Y-galvanometer and then undergoes a cycle of deflection to achieve multiple X-Scan scans, focusing the light beam on the lens, vitreous membrane, fundus and anterior segment interface of the eye tissue to be measured, performing two-dimensional scanning, and realizing the scanning of multiple measurement planes, thereby improving the signal-to-noise ratio. During the focusing process, the system divides the measurement area into three parts: the first part includes the retina, the second part includes the posterior surface of the lens, and the third part includes the anterior surface of the lens and the cornea. By calculating the focusing position of the light beam in advance and changing the position of the lens group, zoom scanning can be achieved. ​

[0051] Specifically, the eye parameters mentioned here do not include the diopter and astigmatism of the eye, and the diopter and astigmatism of the eye need to be measured and calculated through a Hartmann lens group. Assuming that the diopter and astigmatism of the eye are not considered and it is specifically used for measuring the axial length data, then the system can also be simplified to Figure 3 The structural schematic diagram of the high-resolution axial length measurement system for the eye combining high and low coherence measurements with a filtering design shown in the figure.

[0052] The high-resolution axial length measurement system for the eye combining high and low coherence measurements with a filtering design also includes a low coherence interferometric measurement system part and a high coherence auxiliary measurement system part. The difference is that the Hartmann lens group and the two-dimensional galvanometer are cancelled. That is, the axial length data can be directly measured by adjusting the parameters and normal reflection and interference can be generated.

[0053] Because the powers of the first low coherence light and the second low coherence light are equal, the two interference signals enter the balanced photodetector for differential amplification and comparison processing after positive input and negative input respectively, and the phases of the two converted current signals differ by 180 degrees; the differential amplification processing formula is as follows:

[0054] represents the differential amplification output, and respectively represent the two power signals, represents the responsivity of the photodiode at a given wavelength, G is the gain coefficient, and M is the optical gain factor.

[0055] Due to the current signals and having a phase difference of π, after differential processing, the DC signal is eliminated and the amplitude of the AC signal in the output signal is doubled. In addition, due to the use of differential amplification in balanced detection, the common-mode noise signals in the two detectors are expected to be greatly suppressed, and the signal-to-noise ratio of the output signal of the multi-parameter ophthalmic biometric system can be improved.

[0056] Specifically, considering the characteristics of the eye tissue, light with a wavelength of 850 nm has a high transmittance in water, and the power loss caused by the absorption of light with a wavelength of 850 nm in water is small. The main component of the human eye tissue is water, and the water content in the vitreous body exceeds 90%. Therefore, we selected a light source with a wavelength of 850 nm for the low-coherence interference system. The high-coherence light source of this application consists of a 980 nm pump and a wavelength division multiplexer (i.e., wavelength division multiplexer 1). Wavelength division multiplexer 1 is on the optical fiber between the pump and the phase-shifting grating. A photodetector is connected to wavelength division multiplexer 1, and the signal scale is determined by detecting the self-mixing interference signal through the photodetector. It should be noted here that the 980 nm pump will be excited to 1550 nm laser through the erbium-doped fiber. Due to the existence of two different wavelength signals, the 980 nm pump will be paired with a wavelength division multiplexer as an overall light source device, that is, the high-coherence light source. The wavelength division multiplexer 2 connected to the fiber delay line is to cooperate with wavelength division multiplexer 1 to enable the two different wavelength light beams to transmit separately without interference.

[0057] In summary, this system is divided into two parts: time-domain low-coherence measurement and high-coherence interference-assisted measurement, and is connected by a fully fiber-optic structure. This design not only eliminates the work of adjusting the spatial optical path, but also helps to reduce the system size and facilitate integration. At the same time, polarization-maintaining fiber is selected to connect the interference optical path part to avoid the polarization of the transmitted light from changing due to external environmental factors such as temperature, pressure or bending, ensure the polarization states of the two interfering light beams are consistent, and further improve the signal-to-noise ratio of the system.

[0058] Furthermore, considering that Figure 1 the system needs to calculate the diopter and divergence information, a beam splitter prism and a reflector also need to be set in the spatial optical path between the zoom lens and the two-dimensional galvanometer to introduce a part of the light beam reflected by the eyeball into the Hartmann microlens optical path, and the diopter of the eyeball is calculated through the Hartmann array diagram collected by the Hartmann optical path. The following are the steps for collecting the Hartmann array diagram and calculating the diopter parameters: S1. When the wavefront to be measured is received by the microlens array, it is divided into countless sub-wavefronts by the microlens array and converges into a spot array diagram containing the information of the wavefront to be measured on the CCD; The Hartmann wavefront imaging optical path consists of a number of microlenses with the same parameters arranged in an orderly and close manner. When the wavefront to be measured is received by the microlens array, the microlens array divides it into countless sub-wavefronts, and finally converges into a spot array diagram containing the information of the wavefront to be measured on the CCD.

[0059] S2. Based on the spot array diagram, measure the offset of the spot position; The process of measuring the offset of the spot position is to construct five groups of two-dimensional surfaces based on the Zernike polynomial, and then reverse the surface function to measure the diopter and divergence data. In the embodiment of the present application, the surface partial derivative is determined as the position offset of each sampling light point. The polynomial and partial derivative constructed based on the Zernike multi-surface are expressed as follows:

[0060] In the above formula, the polynomial is determined according to the selected quantity and the Zernike polynomial, and the present application will not explain it in detail. Obtain at least one group of light points offset , and invert the coefficient values of the five polynomials according to the reference point coordinates ; The offset is expressed as follows:

[0061] The partial derivative of the sampling point is expressed as follows:

[0062] Among them, represents the wavefront function, and represent the offset value, and are the corresponding derivatives, is the focal length of the lens. In this embodiment, n takes the value of 5, representing 5 surface polynomials.

[0063] S3. Calculate the diopter and divergence data based on the spot position offset.

[0064] Furthermore, after obtaining the above relationship, list the polynomial coefficient identity according to the corresponding relationship between the sampling point partial derivative and the matrix, which is expressed as follows:

[0065] Use the inverse matrix to obtain the Zernike coefficients corresponding to the wavefront aberration, which is expressed as follows:

[0066] Calculate the diopter and divergence through the five coefficients , which is expressed as follows

[0067]

[0068] represents the diopter value, represents the divergence value.

[0069] Figure 4The schematic diagram of the Hartmann array diagram is shown as an example. The light beam is focused on the CCD photosensitive surface, and the Hartmann array diagram is obtained by the CCD data acquisition unit. By comparing with the standard Hartmann array diagram (S=0C=0), accurate refractive power (S) and divergence (C) data can be obtained. Figure 4 The left side in the middle is a standard Hardmann array diagram, and the middle and right sides are Hardmann array diagrams under different degrees of diopter and divergence.

[0070] Based on the above-mentioned high and low coherence measurement and combined with the self-mixing vernier effect and the Hardman microlens group solution, the measurement of the eye axis and refractive power can be achieved, that is, a multi-parameter eye axis measurement system based on the self-mixing vernier effect is formed.

[0071] Considering that the optical path of high and low coherence light sources undergoes multiple conversions and reflections, the electrical signal converted by the balanced detector will have more interference. In order to ensure the accuracy of data processing and detection, this embodiment chooses to filter the data. After comparing various filtering methods, this application finally designed a filtering method for self-adjusting the filtering frequency for interference signal processing, forming a multi-parameter eye axis measurement and data processing system based on the self-mixing cursor effect. The system diagram is similar to Figure 1 The structure is the same, the difference is that the content of post-data sorting and filtering is added. The following are the steps of data processing and filtering: S1, the balanced photodetector performs photoelectric conversion to obtain the original current signal, and converts the time domain into a frequency domain signal through FFT transformation; The mathematical model of Fourier transform is as follows:

[0072] in It's a signal The Fourier transform result represents the frequency domain signal (spectrum); is the time domain signal, w is the angular frequency; the inverse transform formula of Fourier transform is:

[0073] This shows that the frequency domain signal can be restored to the original time domain signal through inverse Fourier transform. Figure 5 Schematic diagrams of the original data time domain diagram and the original data frequency domain diagram are listed.

[0074] Note that symmetric peaks will appear in the Fourier transform (due to complex conjugate symmetry, the Fourier transform will have symmetric peaks at positive and negative frequencies), and the essential reason is that the mathematical properties of real-valued signals result in conjugate symmetry of the spectrum. This phenomenon is very important in both theoretical and practical signal processing (such as spectrum analysis, filter design, etc.) and provides important information about the signal components. In Matlab, the negative frequency part is symmetrically mapped to the positive frequency part, resulting in the two peaks shown in the following figure. In fact, the right peak is the symmetric peak of the negative frequency.

[0075] S2. Design a band-pass FIR filter and set the window according to the interference signal frequency range in combination with the band-pass frequency; Design a band-pass FIR filter according to the interference signal frequency range in combination with the band-pass frequency. The band-pass FIR filter mainly includes band-pass filtering, filter order, and window function. Among them, the most important band-pass filtering contains some important parameters: Lower cut-off frequency ( ): That is, the cut-off frequency of the low-pass filter, representing the lowest frequency that can pass through; Upper cut-off frequency ( ): That is, the cut-off frequency of the high-pass filter, representing the highest frequency that can pass through; Bandwidth (BW): It refers to the frequency band width that the band-pass filter can pass through, usually calculated as:

[0076] Center frequency ( ): It is the center point of the frequency passed by the band-pass filter, usually calculated as:

[0077] In this embodiment, the center frequency of the periodic identification band-pass filtering is the maximum frequency value identified in the frequency-domain signal, and then and are determined according to the bandwidth.

[0078] The frequency response of the band-pass FIR filter can be represented by a frequency response curve, and its shape generally presents a bell-shaped curve (i.e., Gaussian or Lorentz curve), which means that the signal near the center frequency is transmitted most effectively, while the signal outside the cut-off frequency is gradually attenuated. Its mathematical model is:

[0079] where is the frequency response of the band-pass filter.

[0080] In the algorithm implementation, frequency normalization is achieved by the target frequency and the ratios of the upper and lower cut-off frequencies of the band-pass filter to the sampling rate, and then a preliminary band-pass filter is implemented using a window function. Meanwhile, a loop of the upper and lower cut-off frequencies of the band-pass filter and the filter order is introduced, the loop range and its step size are adjusted, and the most suitable FIR filter combination for filtering is found through the loop system. Figure 6 Shows a possible FIR band-pass filtering diagram.

[0081] S3. Extract the envelope and determine the position through Hilbert transform; Extracting the envelope and determining the position through Hilbert transform has significant advantages. Especially when dealing with non-stationary signals, signals with high noise, and complex signals, it can not only accurately extract the instantaneous characteristics of the signal, but also does not require additional preprocessing steps and is not affected by artificial threshold settings. Therefore, in this embodiment, the Hilbert transform is performed on the preliminarily filtered signal and its envelope is extracted.

[0082] Set the real signal of the filtered interference signal as and the imaginary signal of the Hilbert transform is expressed as follows:

[0083] Substantially, it makes the signal generate a 90-degree phase shift, and with as the real part and as the imaginary part, an analytic signal is constructed. The analytic signal constructed according to the real signal and the imaginary signal is expressed as:

[0084] where the modulus length of the analytic signal represents the envelope of the original signal, and its calculation formula is:

[0085] Figure 7 is a possible Hilbert envelope diagram listed. This kind of envelope signal usually represents the amplitude modulation of the signal, that is, the amplitude change of the signal over time.

[0086] S4. Perform Fourier transform on the extracted Hilbert envelope to obtain envelope spectrum data; Performing Fourier transform on the Hilbert envelope, the frequency-domain image of the envelope is as shown in Figure 8 and the desired interference signal frequency range is found.

[0087] S5. Set a low-pass filter for the envelope spectrum data according to the interference signal frequency range, perform secondary filtering, and determine the filtering combination according to the sum of the signal-to-noise ratios.

[0088] This process mainly involves obtaining the interference signals before and after two rounds of filtering, and calculating the signal-to-noise ratio (SNR) based on adjacent interference peaks. Then, the sums of the SNRs calculated under different parameter settings before and after the two rounds of filtering are sorted in descending order, and the filtering combination with the largest sum of SNRs is selected as the target filtering strategy within the period. It specifically includes the upper and lower cut-off frequencies of the band-pass filter, the filter order, and the low-pass frequency of the Hilbert envelope filtering, etc.

[0089] Regarding the determination method of the sum of SNRs, in this application, the SNRs are calculated using two interference peaks respectively, and then the maximum value of the sum of the two items is used as the determination output. This is because if only one interference peak is used as the judgment basis, the frequency selection of the other interference peak will be ignored, resulting in distortion of some signals. Such a determination method can consider the frequency parts of both peaks simultaneously and meet the filtering requirements.

[0090] Based on the above design idea, this application uses this filtering strategy for data processing, while saving calculation time without affecting the filtering effect, increasing the step size, and thus reducing the number of loops. Figure 9 In the shown filtering effect diagram, the calculation time is reduced from 35 s to about 1.5 s, while the sum of the SNRs can still be maintained at about 65 dB, and the SNR of a single peak is maintained at about 32 dB. The position marked in red is the peak on the front surface of the glass, and the position marked in green is the peak on the back surface of the glass. Through high-SNR measurement in terms of structure and filtering processing, an axial eye high-resolution measurement system that combines high and low coherence measurements with filtering design is finally realized.

[0091] In summary, in this solution, the measurement of various parameters of the eye axis is realized through time-domain low-coherence interferometry. The time-domain low-coherence interferometry measurement system consists of a laser, a reference arm, a measurement arm, a delay device, and a detector. The measurement light carries the information to be measured and undergoes low-coherence light interference with the reference light to obtain the interference peaks of each surface of the eye tissue. Based on the time-domain low-coherence interferometry fiber structure system, when the obtained interference signal is taken, it is usually difficult to achieve high-precision biological measurement without processing. Therefore, signal filtering is usually also required. In the filtering technology, through a series of operations such as Fourier transform of the original time-domain data, FIR band-pass filtering, Hilbert transform of the preliminarily processed signal, and low-pass filtering, etc., and at the same time introducing a loop process for frequency and filter order in combination with the calculation of the SNR to select the best filtering combination, finally, short-time and high-SNR processing of the obtained interference signal is realized, and the processing process and results are visualized for easy observation of the changes in the processed signal. Finally, combined with high-coherence interferometry technology, using the high-coherence interference signal as a scale, the precise calibration of the position information of the eye axis tissue can be realized.

[0092] This specific embodiment is only an interpretation of the present invention and does not limit the present invention. After reading this specification, those skilled in the art can make modifications to this embodiment that do not contribute creatively as needed, but as long as it is within the scope of the claims of the present invention, it is protected by the patent law.

Claims

1. A multi-parameter eye axis measurement system based on self-mixing vernier effect, characterized in that: The system comprises: A low-coherence light source, wherein the light beam emitted by the low-coherence light source is divided into a fiber reference light and a fiber measurement light through a first coupler; the fiber measurement light outputs a spatial measurement light through a first circulator and a collimator, and the spatial measurement light focuses the scanning light beam on different parts of the eyeball through a zoom lens; the fiber measurement light enters the fiber delay line through a second circulator and a wavelength division multiplexer, and is output from a reflection end of the second circulator after being reflected by the fiber delay line; A high coherence light source, the light beam emitted by the high coherence light source passes through an erbium-doped optical fiber connected to a phase-shift grating, is reflected by a wavelength division multiplexer and an optical fiber delay line, generates self-mixing interference at the phase-shift grating, and calibrates the low coherence peak spacing; the spatial measurement light is reflected by the eyeball, then passes through a collimator and outputs a first low coherence light from the reflection end of the first circulator, and the sample measurement light outputs a second low coherence light through the reflection end of the second circulator; the two beams of low coherence light generate interference through a second coupler, the interference data is received by a balanced photodetector, and the eyeball data is obtained in combination with the low coherence peak spacing measurement; The spatial light reflected by the eyeball enters the Hardmann microlens group and is focused on the CCD photosensitive surface. The eyeball refractive power is calculated through the collected Hartmann array image.

2. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 1, characterized in that: A two-dimensional galvanometer is arranged in the spatial optical path between the zoom lens and the eyeball, and the spatial light is focused to different areas of the eyeball through the two-dimensional galvanometer; The two-dimensional galvanometer includes an X-mirror and a Y-mirror that are perpendicular to each other. The light passes through the X-mirror to complete one cycle of X-Scan scanning, is reflected to the Y-mirror and then undergoes one cycle of deflection to achieve multiple X-Scan scanning, focusing the light beam on the lens, gelatin membrane, fundus and anterior segment interface of the eye tissue to be tested for two-dimensional scanning.

3. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 2, characterized in that: A beam splitter prism and a reflector are arranged in the spatial optical path between the zoom lens and the two-dimensional galvanometer to introduce part of the light beam reflected by the eyeball into the Hartmann microlens optical path, and the eyeball refractive power is calculated through the Hartmann array image collected by the Hartmann optical path.

4. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 1, characterized in that: The signal strength received by the balanced photodetector I It is expressed as follows: Among them and represent the central wavelength and spectrum half-maximum full width of the low coherence light source, respectively. Indicates receiving interference signal I energy; Represents the DC components of two low-coherence electrical signals and, Indicates the change after interference; Indicates the phase change of the reference light; Measuring signal strength I The process changes the internal attitude of the optical fiber delay line and changes the phase of the reference light. , the eye axis parameters are inferred based on the adjusted posture size.

5. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 4, characterized in that: Based on the periodic change of the interference signal generated by the high coherence light source, the low coherence peak spacing D value is determined by comparing it with the interference peak of the interference signal generated by the low coherence light source, which is expressed as follows: Among them is the wavelength of the high coherence laser, For air at wavelength The refractive index under is the number of complete interference fringes, is the phase difference of the incomplete interference fringes, is the overall phase difference.

6. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to any one of claims 1 to 5, characterized in that: The high coherence light source, phase shift grating, erbium-doped optical fiber, wavelength division multiplexer and optical fiber delay line form a three-mirror cavity model, the phase shift grating is equivalent to the first reflector and the second reflector of the three-mirror cavity model, the optical fiber delay line is equivalent to the third reflector, and the adjustment posture of the optical fiber delay line is determined according to the self-mixing interference formula. , as the signal scale of low coherence interference, is expressed as follows: represents the group refractive index, represents the line width broadening factor, represents the cavity length of the phase-shift grating equivalent resonant cavity, Coupling efficiency, and represent the optical frequency and critical frequency respectively, represents the speed of light, It represents the equivalent external cavity round-trip delay between the phase-shift grating and the fiber delay line.

7. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 6, characterized in that: The high coherence light source is a 980nm pump, and a wavelength division multiplexer is arranged on the optical fiber between the pump and the phase shift grating, and is connected to a photoelectric detector, and the signal scale is determined by detecting the self-mixing interference signal through the photoelectric detector.

8. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 1, characterized in that: The eye refraction is calculated from the acquired Hartmann array, including: When the wavefront to be measured is received by the microlens array, it is divided into countless sub-wavefronts by the microlens array and converged on the CCD into a light spot array image containing the wavefront information to be measured; Calculate the light spot position offset based on the light spot array diagram; The diopter and divergence data are calculated based on the spot position offset.

9. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 8, characterized in that: The process of measuring the spot position offset is to construct five sets of two-dimensional surfaces based on Zernike polynomials, and determine the surface partial derivatives as the position offset of each sampling light point. The surface polynomials and partial derivatives are expressed as follows: Get at least one set of light points The offset , according to the reference point coordinates Invert the coefficients of the five polynomials; offset It is expressed as follows: The partial derivatives at the sampling points are expressed as follows: Among them represents the wavefront function, and Indicates the offset value, and is the corresponding derivative, is the focal length of the lens, and n is 5.

10. The multi-parameter eye axis measurement system based on the self-mixing vernier effect according to claim 9, characterized in that: According to the corresponding relationship between the partial derivatives of the sampling points and the matrix, the polynomial coefficient identities are listed as follows: The Zernike coefficient corresponding to the wavefront aberration is obtained by using the inverse matrix, which is expressed as follows: Through five coefficients Calculate the diopter and divergence, expressed as follows Represents the diopter value, Represents the divergence value.

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